Regulation of the alternative sigma factor σ^E during initiation, adaptation, and shutoff of the extracytoplasmic heat shock response in Escherichia coli

The alternative sigma factor σE is activated in response to stress in the extracytoplasmic compartment of Escherichia coli. Here we show that σE activity increases upon initiation of the stress response by a shift to an elevated temperature (43°C) and remains at that level for the duration of the stress. When the stress is removed by a temperature downshift, σE activity is strongly repressed and then slowly returns to levels seen in unstressed cells. We provide evidence that information about the state of the cell envelope is communicated to σE primarily through the regulated proteolysis of the inner membrane anti-sigma factor RseA, as the degradation rate of RseA is correlated with the changes in σE activity throughout the stress response. However, the relationship between σE activity and the rate of degradation of RseA is complex, indicating that other factors may cooperate with RseA and serve to fine-tune the response.